Fuel compositions

ABSTRACT

Use of a Fischer-Tropsch derived fuel component, in a fuel composition, is provided reducing the tendency of the composition to dissolve metals; increasing its thermal stability; reducing the concentration of a metal deactivator, antioxidant or detergent additive in the composition; or increasing the storage stability of the composition. The composition is preferably a diesel fuel composition.

FIELD OF THE INVENTION

The present invention relates to certain types of fuel compositions.

BACKGROUND OF THE INVENTION

The thermal stability of middle distillate fuels has traditionally beena cause for concern in the aviation industry. Aviation fuels (kerosenefractions) are subjected to high levels of thermal stress during use.

For automotive diesel fuels, thermal stability has historically beenless of a concern. However, trends in modern engine design, to complywith ever tightening emissions legislation, may change this. New commonrail or unit injectors subject fuels to much more severe conditions thanmore traditional diesel engines, for example pressures of up to 2000 barand temperatures above 100° C. Under these conditions, instabilityreactions are much more likely to occur.

Fuel thermal instability reactions are recognised to result from acombination of hydrocarbon oxidation reactions and interactions betweenpolar species present in the fuel. These processes can be affected bytwo competing chemical trends. On the one hand, increasingly low fuelsulphur levels are resulting in lower levels of polar species(typically, the processes used to remove sulphur from a fuel will alsoresult in a reduction in the level of other polar species such asnitrogen containing compounds and oxygenates), and hence a lower levelof natural antioxidancy; this in turn can increase the extent to whichoxidation reactions can occur, in particular when a fuel is subjected tothermal stress. On the other hand, polar species are often the bridgingmoieties which form fuel lacquers in thermal instability reactions;thus, lower levels of polar species can to some extent help to reducethe number of thermal instability reactions occurring.

Poor thermal stability in a fuel will result in an increase in theproducts of thermal instability reactions such as gums, lacquers andother insoluble components. These in turn can block engine filters, foulfuel injectors and valves and hence be detrimental to engine efficiencyand emissions control. Fuel instability is also thought to lead toincreased soot production in engine exhausts, which could lead tooverloading of particulate traps. Thus, it is desirable for a fuel tohave as high as possible a thermal stability, in particular in systems(such as common rail or unit injector diesel engines, or indeed aircraftengines) in which the fuel is subjected to a significant level ofthermal stress.

It has also been seen, in the aviation industry, that thermalinstability of a fuel can be exacerbated by the presence of tracecatalytic metals—for example copper—which can occur if the fuel is ableto dissolve such metals from the engine hardware, or from storage tanksor transportation equipment.

SUMMARY OF THE INVENTION

Accordingly there is provided in one embodiment a method for formulatinga fuel composition, which method comprising blending together anon-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuelcomponent, optionally with other fuel components, in which the JFTOTbreakpoint of such fuel composition is greater than 300° C.

There is provided in another embodiment the method for formulating afuel composition, which method comprising blending together anon-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuelcomponent, optionally with other fuel components, the peroxide level ofsuch fuel composition is 10 mg/kg or less after a period of storage of 8weeks under storage temperature of at least 40° C.

Further a method of operating a fuel composition made by such methodsare provided.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a fuel composition, and/or components foruse in a fuel composition, which can overcome or at least mitigate theabove described problems.

It has been found that a Fischer-Tropsch derived fuel component can havea much lower tendency to dissolve metals, in particular catalytic metalssuch as copper, than do conventional petroleum derived fuels. This inturn has been shown to result in a higher thermal stability. Moreover,Fischer-Tropsch derived fuel components of the invention appear to havehigh intrinsic thermal stabilities compared to petroleum derived fuels,thereby increasing the thermal stability of the fuel composition.

That this is possible is not necessarily obvious, since Fischer-Tropschderived fuel components are well known to contain low levels of polarspecies, which might be expected to lead to an increased susceptibilityto oxidation and hence a poorer thermal stability.

A certain level of thermal stability may be desirable in order for afuel composition to meet current fuel specifications, and/or to complywith local regulations, and/or to satisfy consumer demand, and/or toensure efficient or at least adequate operation of a fuel consumingsystem to be run on the composition. According to the present invention,such standards may still be achievable, due at least in part to the useof the Fischer-Tropsch derived fuel component.

Since it may be desirable to include a Fischer-Tropsch derived componentin a fuel composition for other reasons, for example to reduce emissionsfrom a fuel-consuming system (typically an engine) running on the fuelcomposition, or to reduce the level of sulphur and/or aromatics and/orother polar components in the composition, the ability to use aFischer-Tropsch component for the additional purpose of reducing theuptake by the composition of catalytic metals, and improving the thermalstability of the composition, can provide significant formulationadvantages.

The present invention may additionally or alternatively be used toadjust any property of the fuel composition which is equivalent to orassociated with either thermal stability or tendency to dissolve metals,for example storage stability (as described below); tendency to producedegradation products such as gums, lacquers and other deposits; tendencyto discolour (which may in turn be due to the formation of degradationproducts); and/or detrimental effect on an engine or otherfuel-consuming system, for instance on its efficiency and/or emissionsand/or on components of the system such as its catalytic system.

In the context of the present invention, “use” of a Fischer-Tropschderived component in a fuel composition means incorporating thecomponent into the composition, optionally as a blend (i.e. a physicalmixture) with one or more other fuel components. In one embodiment ofthe present invention, the Fischer-Tropsch derived fuel component may bethe only fuel component present in the composition, optionally with oneor more fuel additives. The Fischer-Tropsch derived component willconveniently be incorporated before the fuel composition is introducedinto an engine or other system which is to be run on the composition.Instead or in addition the use of the Fischer-Tropsch derived fuelcomponent may involve running a fuel-consuming system, typically adiesel engine, on a fuel composition containing or consisting of theFischer-Tropsch component, typically by introducing the composition intoa combustion chamber of an engine.

“Use” of a Fischer-Tropsch derived fuel component in the ways describedabove may also embrace supplying such a component together withinstructions for its use in a fuel composition to achieve any of thepurposes described above, for instance to reduce the tendency of thecomposition to dissolve metals and increase its thermal stability. TheFischer-Tropsch derived fuel component may itself be supplied as part ofa formulation suitable for and/or intended for use as a fuel additive,in which case the Fischer-Tropsch component may be included in such aformulation for the purpose of influencing its effects on the metalsolubilisation capability of a fuel composition, and its the thermalstability.

Thus, the Fischer-Tropsch derived component may be incorporated into anadditive formulation or package along with one or more fuel additivesselected for instance from detergents, lubricity enhancing additives,ignition improvers and static dissipaters.

The fuel composition used in the present invention may be for example anaphtha, kerosene or diesel fuel composition, in particular a keroseneor diesel fuel composition. It may be a middle distillate fuelcomposition, such as a heating oil, an industrial gas oil, an automotivediesel fuel, a distillate marine fuel or a kerosene fuel such as anaviation fuel or heating kerosene. It may be for use in an engine suchas an automotive engine or an aircraft engine. In one embodiment it isfor use in an internal combustion engine; for instance it may be anautomotive fuel composition, such as a diesel fuel composition which issuitable for use in an automotive diesel (compression ignition) engine.

As described above, the Fischer-Tropsch derived fuel may be the onlyfuel component in a composition prepared according to the presentinvention. Alternatively, such a fuel composition may contain, inaddition to the Fischer-Tropsch derived fuel component, one or morenon-Fischer-Tropsch derived base fuels such as petroleum derived basefuels. In this case the fuel composition prior to incorporation of theFischer-Tropsch derived component may contain a major proportion of, orconsist essentially or entirely of, a base fuel such as a distillatehydrocarbon base fuel. A “major proportion” means typically 80% v/v orgreater, or 90 or 95% v/v or greater, or even 98 or 99 or 99.5% v/v orgreater. Such a base fuel may for example be a naphtha, kerosene ordiesel fuel, preferably a kerosene or diesel fuel, such as a dieselfuel.

A naphtha base fuel will typically boil in the range from 25 to 175° C.A kerosene base fuel will typically boil in the range from 140 to 260°C. A diesel base fuel will typically boil in the range from 150 to 400°C.

The base fuel may in particular be a middle distillate base fuel, inparticular a diesel base fuel, and in this case it may itself comprise amixture of middle distillate fuel components (components typicallyproduced by distillation or vacuum distillation of crude oil), or offuel components which together form a middle distillate blend. Middledistillate fuel components or blends will typically have boiling pointswithin the usual middle distillate range of 125 to 550° C. or 140 to400° C.

A diesel base fuel may be an automotive gas oil (AGO). Typical dieselfuel components comprise liquid hydrocarbon middle distillate fuel oils,for instance petroleum derived gas oils. Such base fuel components maybe organically or synthetically derived. They will typically haveboiling points within the usual diesel range of 140 or 150 to 400 or550° C., depending on grade and use. They will typically have densitiesfrom 0.75 to 1.0 g/cm³, preferably from 0.8 to 0.9 or 0.86 g/cm³, at 15°C. (IP 365) and measured cetane numbers (ASTM D613) of from 35 to 80,more preferably from 40 to 75 or 70. Their initial boiling points willsuitably be in the range 150 to 230° C. and their final boiling pointsin the range 290 to 400° C. Their kinematic viscosity at 40° C. (ASTMD445) might suitably be from 1.5 to 4.5 mm²/s.

Such fuels are generally suitable for use in a compression ignition(diesel) internal combustion engine, of either the indirect or directinjection type.

A diesel fuel composition which results from carrying out the presentinvention may also fall within these general specifications. It may forinstance comply with applicable current standard specification(s) suchas for example EN 590 (for Europe) or ASTM D975 (for the USA). By way ofexample, the fuel composition may have a density from 0.82 to 0.845g/cm³ at 15° C.; a T₉₅ boiling point (ASTM D86) of 360° C. or less; acetane number (ASTM D613) of 51 or greater; a kinematic viscosity (ASTMD445) from 2 to 4.5 mm²/s at 40° C.; a sulphur content (ASTM D2622) of50 mg/kg or less; and/or a polycyclic aromatic hydrocarbons (PAH)content (IP 391 (mod)) of less than 11%. Relevant specifications mayhowever differ from country to country and from year to year and maydepend on the intended use of the fuel composition.

A petroleum derived gas oil may be obtained by refining and optionally(hydro)processing a crude petroleum source. It may be a single gas oilstream obtained from such a refinery process or a blend of several gasoil fractions obtained in the refinery process via different processingroutes. Examples of such gas oil fractions are straight run gas oil,vacuum gas oil, gas oil as obtained in a thermal cracking process, lightand heavy cycle oils as obtained in a fluid catalytic cracking unit andgas oil as obtained from a hydrocracker unit. Optionally a petroleumderived gas oil may comprise some petroleum derived kerosene fraction.

Such gas oils may be processed in a hydrodesulphurisation (HDS) unit soas to reduce their sulphur content to a level suitable for inclusion ina diesel fuel composition. This also tends to reduce the content ofother polar species such as nitrogen-containing species.

In the present invention, a base fuel may be or contain a so-called“biofuel” component such as a vegetable oil or vegetable oil derivative(e.g. a fatty acid ester, in particular a fatty acid methyl ester) oranother oxygenate such as an acid, ketone or ester. Such components neednot necessarily be bio-derived.

The fuel composition to which the present invention is applied may havea sulphur content of 1000 mg/kg or less. It may have a low or ultra lowsulphur content, for instance at most 500 mg/kg, or at most 350 mg/kg,suitably no more than 100 or 50 or 10 or even 5 mg/kg, of sulphur.

By “Fischer-Tropsch derived” is meant that a fuel component is, orderives from, a synthesis product of a Fischer-Tropsch condensationprocess. A Fischer-Tropsch derived fuel may also be referred to as a GTL(Gas-to-Liquids) fuel. The term “non-Fischer-Tropsch derived” may beconstrued accordingly.

It is known to include such components in fuel compositions; inparticular, Fischer-Tropsch derived gas oils have been included inautomotive diesel fuels. What has not been appreciated before, to ourknowledge, is their ability to influence the metal solubilisationcapacity of a fuel composition and in turn its thermal stability.

The Fischer-Tropsch reaction converts carbon monoxide and hydrogen intolonger chain, usually paraffinic, hydrocarbons:

n(CO+2H₂)═(—CH₂—)_(n) +nH₂O+heat,

in the presence of an appropriate catalyst and typically at elevatedtemperatures (e.g. 125 to 300° C., preferably 175 to 250° C.) and/orpressures (e.g. 5 to 100 bar, preferably 12 to 50 bar). Hydrogen:carbonmonoxide ratios other than 2:1 may be employed if desired.

The carbon monoxide and hydrogen may themselves be derived from organicor inorganic, natural or synthetic sources, typically either fromnatural gas or from organically derived methane. The gases which areconverted into liquid fuel components using such processes can ingeneral include natural gas (methane), LPG (e.g. propane or butane),“condensates” such as ethane, synthesis gas (CO/hydrogen) and gaseousproducts derived from coal, biomass and other hydrocarbons.

Gas oil, naphtha and kerosene products may be obtained directly from theFischer-Tropsch reaction, or indirectly for instance by fractionation ofFischer-Tropsch synthesis products or from hydrotreated Fischer-Tropschsynthesis products. Hydrotreatment can involve hydrocracking to adjustthe boiling range (see, e.g., GB-B-2077289 and EP-A-0147873) and/orhydroisomerisation which can improve cold flow properties by increasingthe proportion of branched paraffins. EP-A-0583836 describes a two stephydrotreatment process in which a Fischer-Tropsch synthesis product isfirstly subjected to hydroconversion under conditions such that itundergoes substantially no isomerisation or hydrocracking (thishydrogenates the olefinic and oxygen-containing components), and then atleast part of the resultant product is hydroconverted under conditionssuch that hydrocracking and isomerisation occur to yield a substantiallyparaffinic hydrocarbon fuel. The desired gas oil fraction(s) maysubsequently be isolated for instance by distillation.

Other post-synthesis treatments, such as polymerisation, alkylation,distillation, cracking-decarboxylation, isomerisation andhydroreforming, may be employed to modify the properties ofFischer-Tropsch condensation products, as described for instance in U.S.Pat. No. 4,125,566 and U.S. Pat. No. 4,478,955.

Typical catalysts for the Fischer-Tropsch synthesis of paraffinichydrocarbons comprise, as the catalytically active component, a metalfrom Group VIII of the periodic table, in particular ruthenium, iron,cobalt or nickel. Suitable such catalysts are described for instance inEP-A-0583836 (pages 3 and 4).

An example of a Fischer-Tropsch based process is the SMDS (Shell MiddleDistillate Synthesis) described by van der Burgt et al in “The ShellMiddle Distillate Synthesis Process”, paper delivered at the 5thSynfuels Worldwide Symposium, Washington D.C., November 1985 (see alsothe November 1989 publication of the same title from Shell InternationalPetroleum Company Ltd, London, UK). This process (also sometimesreferred to as the Shell “Gas-To-Liquids” or “GTL” technology) producesmiddle distillate range products by conversion of a natural gas(primarily methane) derived synthesis gas into a heavy long chainhydrocarbon (paraffin) wax which can then be hydroconverted andfractionated to produce liquid transport fuels such as the gas oilsuseable in diesel fuel compositions. A version of the SMDS process,utilising a fixed bed reactor for the catalytic conversion step, iscurrently in use in Bintulu, Malaysia and its gas oil products have beenblended with petroleum derived gas oils in commercially availableautomotive fuels.

Gas oils, naphthas and kerosenes prepared by the SMDS process arecommercially available for instance from Shell companies. Furtherexamples of Fischer-Tropsch derived gas oils are described inEP-A-0583836, EP-A-1101813, WO-A-97/14768, WO-A-97/14769, WO-A-00/20534,WO-A-00/20535, WO-A-00/11116, WO-A-00/11117, WO-A-01/83406,WO-A-01/83641, WO-A-01/83647, WO-A-01/83648 and U.S. Pat. No. 6,204,426.

By virtue of the Fischer-Tropsch process, a Fischer-Tropsch derived fuelhas essentially no, or undetectable levels of, sulphur and nitrogen.Compounds containing these heteroatoms tend to act as poisons forFischer-Tropsch catalysts and are therefore removed from the synthesisgas feed. This reduction in the level of polar species might be expectedto reduce the thermal stability of a Fischer-Tropsch derived fuel, whichmakes the present invention all the more surprising.

Further, the Fischer-Tropsch process as usually operated produces no orvirtually no aromatic components, which again might be expected toreduce the thermal stability of the resultant fuel. The aromaticscontent of a Fischer-Tropsch derived fuel, suitably determined by ASTMD4629, will typically be below 1% w/w, preferably below 0.5% w/w andmore preferably below 0.2 or 0.1% w/w.

Generally speaking, Fischer-Tropsch derived fuels have relatively lowlevels of polar components, in particular polar surfactants, forinstance compared to petroleum derived fuels. Such polar components mayinclude for example oxygenates, and sulphur- and nitrogen-containingcompounds. A low level of sulphur in a Fischer-Tropsch derived fuel isgenerally indicative of low levels of both oxygenates andnitrogen-containing compounds, since all are removed by the sametreatment processes.

Where a Fischer-Tropsch derived fuel component is a naphtha fuel, itwill be a liquid hydrocarbon distillate fuel with a final boiling pointof typically up to 220° C. or preferably of 180° C. or less. Its initialboiling point may be higher than 25° C., in cases higher than 35° C. Itscomponents (or the majority, for instance 95% w/w or greater, thereof)are typically hydrocarbons having 5 or more carbon atoms; they areusually paraffinic.

In the context of the present invention, a Fischer-Tropsch derivednaphtha fuel may have a density of from 0.67 to 0.73 g/cm³ at 15° C.and/or a sulphur content of 5 mg/kg or less, preferably 2 mg/kg or less.It may contain 95% w/w or greater of iso- and normal paraffins,preferably from 20 to 98% w/w or greater of normal paraffins. It may bethe product of a SMDS process, suitable features of which may be asdescribed below in connection with Fischer-Tropsch derived gas oils.

A Fischer-Tropsch derived kerosene fuel is a liquid hydrocarbon middledistillate fuel with a distillation range suitably from 140 to 260° C.,preferably from 145 to 255° C., more preferably from 150 to 250° C. orfrom 150 to 210° C. It will have a final boiling point of typically from190 to 260° C., for instance from 190 to 210° C. for a typical“narrow-cut” kerosene fraction or from 240 to 260° C. for a typical“full-cut” fraction. Its initial boiling point is preferably from 140 to160° C., more preferably from 145 to 160° C.

A Fischer-Tropsch derived kerosene fuel may have a density of from 0.730to 0.760 g/cm³ at 15° C.—for instance from 0.730 to 0.745 g/cm³ for anarrow-cut fraction and from 0.735 to 0.760 g/cm³ for a full-cutfraction. It preferably has a sulphur content of 5 mg/kg or less. It mayhave a cetane number of from 63 to 75, for example from 65 to 69 for anarrow-cut fraction or from 68 to 73 for a full-cut fraction. It may bethe product of a SMDS process, suitable features of which may be asdescribed below in connection with Fischer-Tropsch derived gas oils.

A Fischer-Tropsch derived gas oil should be suitable for use as a dieselfuel, ideally as an automotive diesel fuel; its components (or themajority, for instance 95% w/w or greater, thereof) should thereforehave boiling points within the typical diesel fuel (“gas oil”) range,i.e. from about 150 to 400° C. or from 170 to 370° C. It will suitablyhave a 90% w/w distillation temperature of from 300 to 370° C.

A Fischer-Tropsch derived gas oil will typically have a density from0.76 to 0.79 g/cm³ at 15° C.; a cetane number (ASTM D613) greater than70, suitably from 74 to 85; a kinematic viscosity (ASTM D445) from 2 to4.5, such as from 2.5 to 4.0 or from 2.5 to 3.7, mm²/s at 40° C.; and/ora sulphur content (ASTM D2622) of 5 mg/kg or less, in cases of 2 mg/kgor less.

A Fischer-Tropsch derived fuel component used in the present inventionmay for instance be a product prepared by a Fischer-Tropsch methanecondensation reaction using a hydrogen/carbon monoxide ratio of lessthan 2.5, or of less than 1.75, or from 0.4 to 1.5, and suitably using acobalt containing catalyst. It may have been obtained from ahydrocracked Fischer-Tropsch synthesis product (for instance asdescribed in GB-B-2077289 and/or EP-A-0147873), or a product from atwo-stage hydroconversion process such as that described in EP-A-0583836(see above). In the latter case, suitable features of thehydroconversion process may be as disclosed at pages 4 to 6, and in theexamples, of EP-A-0583836.

Suitably, a Fischer-Tropsch derived fuel component used in the presentinvention is a product prepared by a low temperature Fischer-Tropschprocess, by which is meant a process operated at a temperature of 250°C. or lower, such as from 125 to 250° C. or from 175 to 250° C., asopposed to a high temperature Fischer-Tropsch process which mighttypically be operated at a temperature of from 300 to 350° C.

Suitably, in accordance with the present invention, a Fischer-Tropschderived fuel component will consist of at least 70% w/w, or at least 80%w/w, or at least 90 or 95 or 98% w/w, or at least 99 or 99.5 or even99.8% w/w, of paraffinic components, in particular iso- and normalparaffins. The weight ratio of iso-paraffins to normal paraffins willsuitably be greater than 0.3 and may be up to 12; suitably it is from 2to 6. The actual value for this ratio will be determined, in part, bythe hydroconversion process used to prepare the gas oil from theFischer-Tropsch synthesis product.

The olefin content of the Fischer-Tropsch derived fuel component issuitably 0.5% w/w or lower. Its aromatics content is suitably 0.5% w/wor lower.

In accordance with the present invention, the Fischer-Tropsch derivedfuel component may be for example a naphtha, kerosene or diesel (gasoil) component, suitably a kerosene or diesel component, such as adiesel component.

A fuel composition prepared according to the present invention maycontain a mixture of two or more Fischer-Tropsch derived fuelcomponents.

The concentration of the Fischer-Tropsch derived fuel component, in acomposition prepared according to the present invention, may be 1% v/vor greater, such as 2 or 5 or 10 or 15% v/v or greater, for example 20or 25 or 30 or 40 or 50% v/v or greater. It may be up to 100% v/v (i.e.the fuel is entirely Fischer-Tropsch derived), or it may be up to 99 or98 or 95 or 90 or 80% v/v, in cases up to 75 or 60 or 50% v/v. Suitablythe proportion of Fischer-Tropsch derived fuel component(s) in thecomposition is up to 40 or in cases 30% v/v, or up to 25 or 20 or 15%v/v; for example it may be from 5 to 30% v/v.

The Fischer-Tropsch derived fuel component may be used in the fuelcomposition for one or more other purposes in addition to the desire toreduce metal dissolution capability and increase thermal stability, forinstance to reduce emissions from a fuel-consuming system (typically anengine) running on the fuel composition, and/or to reduce the level ofsulphur and/or aromatics and/or other polar components in thecomposition. Thus the present invention can be used to optimise theproperties and performance of a fuel composition in a number of ways,and can therefore provide additional flexibility in fuel formulation.

The tendency of a fuel composition to dissolve metals refers to itstendency or ability to take up a metal from a metal surface, typically apart of an engine or other fuel consuming system, with which thecomposition is placed into contact, suitably during normal operation ofthe fuel consuming system. This tendency may suitably be assessed bymeasuring the amount of the relevant metal in the fuel composition aftercontact with the surface for a given period of time and under specifiedconditions, for instance as described in Example 1 below.

The test conditions may be designed to mimic those to which the fuelcomposition might be subjected when used in a fuel consuming system suchas an internal combustion engine. They may for example involve increasedtemperature, for instance of 30° C. or higher or of 40° C. or higher,such as from 30 to 40° C. (to mimic conditions in a typical vehicle fueltank during fuel recycling from an engine); from 40 to 80° C. (to mimicconditions in the high pressure pump and rail of a common rail injectionsystem); from 80 to 100° C. (to mimic conditions in typical vehicleengine fuel injectors which are in thermal contact with the engineblock); from 100 to 150° C. (to mimic conditions to which a fuel issubjected when close to an injector nozzle); and/or up to 250° C. (as inaccelerated tests, such as at the metal tube surface in the JFTOT testdescribed in the examples below).

The test conditions may involve a pressure from atmospheric (to mimicstorage conditions in a typical fuel tank) to around 1000 or 1500 oreven 2000 bar (to which a fuel composition might be exposed in a typicalcommon rail diesel engine injection system). Suitably the testconditions involve increased pressure, i.e. a pressure aboveatmospheric, for example a pressure of up to 50 bar, such as around 33.3bar as in the JFTOT test used in the examples below.

The present invention may be used to reduce the tendency of the fuelcomposition to dissolve any one or more metals. The metal may be acatalytically active metal, such as copper, iron, zinc, lead, silver,chromium, aluminium, magnesium, nickel or tin, in particular iron orcopper which may be present in fuel storage systems. Its dissolutioninto the fuel composition may be from a metal or metal-containing (forinstance a metal alloy) body, including a body containing a metal salt(for example, an oxide or sulphide or a corrosion product such as rust).Such a metal may be present in the fuel composition in an elemental orionic (which includes complexed) form.

In the context of the first aspect of the present invention, the term“reducing” embraces any degree of reduction, including reduction tozero. The reduction may for instance result in the fuel compositioncontaining at least 10% less of the relevant metal, after contact with ametal-containing surface, than would the same composition prior toincorporation of the Fischer-Tropsch derived fuel component, ifcontacted with the same surface for the same period of time and underthe same conditions. This figure may in cases be at least 25 or 40 or50%, in cases at least 60 or 70 or even 80%.

The reduction may be as compared to the metal dissolving tendency whichthe fuel composition would otherwise have exhibited prior to therealisation that a Fischer-Tropsch derived fuel component could be usedin the way provided by the present invention, and/or that of anotherwise analogous fuel composition intended (e.g. marketed) for use inan analogous context, prior to adding a Fischer-Tropsch derived fuelcomponent to it in accordance with the present invention.

The thermal stability of a fuel composition may in the present contextbe regarded as its thermal oxidation stability. It may be measured inany suitable manner, such as using the Jet Fuel Thermal Oxidation Tester(JFTOT) method, for instance as described in Examples 2 and 3 below.Thermal stability may be assessed with reference to a maximumtemperature at which the fuel still fulfils specified criteria, as forexample the JFTOT “breakpoint”.

Alternatively or additionally, the thermal oxidation stability of a fuelcomposition may be assessed by measuring the change in peroxide numberof the composition (for example, using the standard test method ASTMD3703) following subjection to a specific (typically high temperature)event or condition.

The term “increasing”, in the context of thermal stability, embraces anydegree of increase. The increase may for instance result in the fuelcomposition having a JFTOT breakpoint which is at least 5% higher thanprior to incorporation of the Fischer-Tropsch derived fuel component.This figure may in cases be at least 8 or 10 or 25 or 50%. Again theincrease may be as compared to the thermal stability of the fuelcomposition prior to the realisation that a Fischer-Tropsch derived fuelcomponent could be used in the way provided by the present invention,and/or of an otherwise analogous fuel composition intended (e.g.marketed) for use in an analogous context, prior to adding aFischer-Tropsch derived fuel component to it in accordance with thepresent invention.

In absolute terms, the JFTOT breakpoint of a fuel composition whichresults from carrying out the present invention may be greater than 300or 350° C., or it may be 360° C. or greater, such as 370 or 380° C. orhigher. Ideally the fuel composition has a JFTOT breakpoint within theseranges even when it contains up to 10 or even 15 ppbw (parts per billionby weight) of a dissolved metal such as copper.

Prior to incorporation of the Fischer-Tropsch derived component, thefuel composition may for instance have a JFTOT breakpoint of 350° C. orless, or 300° C. or less, or 250° C. or less.

The thermal stability of a fuel composition may reduce during itsstorage and/or use, for example due to dissolution of one or more metalsfrom a fuel consuming system in which it is stored or used. According tothe present invention, a Fischer-Tropsch derived fuel component may beused in a fuel composition for the purpose of reducing the tendency of afuel composition to suffer such a reduction in thermal stability duringstorage or use. It has been found that not only is a Fischer-Tropschderived fuel component likely to dissolve less metal than other, forexample petroleum derived, fuels, but that on uptake of dissolved metalit may suffer from less of a reduction in thermal stability than would anon-Fischer-Tropsch derived fuel.

A fuel composition to which the present invention is or has been appliedmay contain other standard fuel additives, many of which are known andreadily available. The total additive content in the fuel compositionmay suitably be from 50 to 10000 mg/kg, such as below 5000 mg/kg.

Additives often included in fuel compositions are metal deactivators andcorrosion inhibitors. As a result of carrying out the present invention,however, lower levels of such additives may be needed as the compositionis likely to be less aggressive towards metals during use.

Thus, according to a second aspect, the present invention provides theuse of a Fischer-Tropsch derived fuel component, in a fuel composition,for the purpose of reducing the concentration of a metal deactivator inthe composition. The concentration of a corrosion inhibitor may also bereduced. The metal deactivator or corrosion inhibitor may be of anytype. “Reducing” its concentration may embrace any degree of reduction,including to zero.

Another type of additive often included in fuel compositions is ananti-oxidant. Again as a result of carrying out the present invention,lower levels of such additives may be needed as the composition has ahigher thermal oxidation stability.

Thus, according to a third aspect, the present invention provides theuse of a Fischer-Tropsch derived fuel component, in a fuel composition,for the purpose of reducing the concentration of an antioxidant in thecomposition. The antioxidant may be of any type. “Reducing” itsconcentration may embrace any degree of reduction, including to zero.

Detergent additives are also often included in fuel compositions. Thepresent invention may reduce the need for such additives, by reducingthe level of deposits which are formed (and which therefore need to bedispersed) during storage and use of a fuel composition.

Thus, according to a fourth aspect, the present invention provides theuse of a Fischer-Tropsch derived fuel component, in a fuel composition,for the purpose of reducing the concentration of a detergent additive inthe composition. The detergent additive may be of any type. “Reducing”its concentration may embrace any degree of reduction, including tozero.

A fifth aspect of the present invention provides a method forformulating a fuel composition, which method involves blending togethera non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derivedfuel component, optionally with other fuel components (such as fueladditives), for the purpose of reducing the tendency of the blend todissolve metals. The present invention also provides use in a fuelcomposition of a blend of a non-Fischer-Tropsch derived base fuel and aFischer-Tropsch derived fuel component, optionally with other fuelcomponents (such as fuel additives), for the purpose of reducing thetendency of the blend to dissolve metals. The thermal stability of theblend may also be increased.

The methods of the present invention may be used for the purpose ofachieving a desired target (typically minimum) thermal stability for thefuel composition. This target may be a JFTOT breakpoint within theranges quoted above.

According to a sixth aspect, the present invention provides a method ofoperating a fuel consuming system, which method involves introducinginto the system a fuel composition prepared in accordance with any oneof the first to the fifth aspects of the present invention. The fuelcomposition may be introduced for one or more of the purposes describedabove in connection with the first to the fifth aspects of the presentinvention, in particular to reduce the amount of metal which it takes upfrom parts of the system with which it comes into contact, and toimprove the thermal stability of the fuel composition, and/or to reduceoccurrence of effects associated (whether directly or indirectly) withfuel thermal instability, for example filter blocking or valve orinjector fouling, or loss of system efficiency or emissions control.

In the context of the present invention, a “fuel consuming system”includes a system which transports (for example by pumping) or stores afuel composition, as well as a system which runs on (and hence combusts)a fuel composition.

The system may in particular be an engine, such as an automotive oraircraft engine, in which case the method involves introducing therelevant fuel composition into a combustion area of the engine. It maybe an internal combustion engine, and/or a vehicle which is driven by aninternal combustion engine. The engine is preferably a compressionignition (diesel) engine. Such a diesel engine may be of the directinjection type, for example of the rotary pump, in-line pump, unit pump,electronic unit injector or common rail type, or of the indirectinjection type. It may be a heavy or a light duty diesel engine.

The present invention may be of particular use where the fuel consumingsystem is of the type which subjects a fuel composition to significantlevels of thermal stress, for instance one which subjects a fuelcomposition to pressures in excess of 1000 or 1500 or 2000 bar and/orone which subjects a fuel composition to operating temperatures of 100°C. or greater or of 120 or 140° C. or greater. The fuel consuming systemmay for instance involve high pressure fuel injection.

According to a seventh aspect, the present invention provides a fuelcomposition preparable by, or which has been prepared by, a methodaccording to any one of the first to the fifth aspects of the presentinvention.

In addition to relatively high intrinsic thermal stabilities,Fischer-Tropsch derived fuels are also now believed to have relativelyhigh storage stabilities (typically, stability against oxidation),compared for instance to petroleum derived fuels. Moreover, sincedissolved metals are also believed to impact on storage stability, therelatively low tendency of a Fischer-Tropsch derived fuel component todissolve metals may also help to improve the storage stability of a fuelcomposition containing such a component.

Thus, according to an eighth aspect of the present invention, there isprovided the use of a Fischer-Tropsch derived fuel component, in a fuelcomposition, for the purpose of increasing the storage stability of thecomposition.

All hydrocarbon fuels degrade to some extent during storage, thedegradation rate depending on their composition and storage conditions.When it does occur, storage instability manifests itself as a darkeningin the colour of the fuel and the formation of a fine organic sludge. Ifthe fuel is subsequently stirred up, for instance during tank filling,this sludge is dispersed and can cause filter blockages if the fuel isused before the sludge has resettled.

Fuel instability may also lead to undesirable deposits in thepre-combustion and combustion areas of fuel injection systems, and/or toincreased soot production in engine exhausts which in turn may lead tooverloading of particulate traps.

Poor oxidation stability during storage or thermal stressing is known tolead to the accumulation of peroxides in a fuel. These in turn areassociated with a number of undesirable side effects. For example,peroxides can attack and degrade elastomeric parts within an engine orother system in which the fuel is being used. Oxidation intermediatescan also react with other species present in the fuel (for example,polar compounds) to produce gums and sludges, which in turn can blockengine filters, foul fuel injectors and valves and hence be detrimentalto engine efficiency and emissions control. Moreover, peroxides arethemselves corrosive to metals, and their breakdown products acidic;thus higher peroxide levels can lead to increased corrosion within afuel consuming system.

The storage stability of in particular automotive diesel fuels is likelyto become increasingly problematic as fuel sulphur levels decrease. Thepresence of sulphur-containing species in a fuel can contribute a degreeof natural antioxidancy, but as sulphur levels fall to meet with evertightening emissions legislation (the adoption in 1996 of a lowsulphur—0.05% w/w or less—specification for European automotive gasoils, followed by subsequent increasing pressure to reduce sulphurlevels in cases to less than 10 mg/kg), there has been increasingconcern about the impact this might have on the oxidation stability ofthe fuels. At sulphur levels of 50 mg/kg or less it is unlikely thatfuels will possess sufficient natural antioxidancy to protect againstoxidation reactions during typical storage periods.

The eighth aspect of the present invention provides the use, in a fuelcomposition, of a Fischer-Tropsch derived fuel component, for thepurpose of improving the storage stability of the composition.

It has been found that a Fischer-Tropsch derived fuel component canaccumulate significantly lower levels of peroxides, on storage, than aconventional petroleum derived fuel. This implies a higher storagestability for the Fischer-Tropsch derived fuel.

Moreover, it is believed that not only the presence of naturalantioxidancy (for instance, due to sulphur containing species), but alsothe hydrocarbon structure, can be relevant to the oxidation stability ofa fuel. The ability to form stable hydrocarbon radicals can promote theradical driven autoxidation process and hence decrease storagestability. Radical stability is believed to be greater for aromaticspecies than for cyclic and iso-paraffins, and lower still for normalparaffins. Thus, it is now believed that autoxidation processes couldproceed more readily in a fuel with higher levels of for instancearomatic components, with a consequent detrimental effect on itsoxidation stability.

The balance between the two competing influences on oxidationstability—on the one hand the presence of polar species contributing tothe natural antioxidancy of a fuel and on the other the ability ofspecies such as aromatics to help promote radical driven autoxidationprocesses—is not yet fully understood. It is not thereforestraightforward to predict the oxidation stability of any given fuelcomponent.

Fischer-Tropsch derived fuels tend to contain relatively low levels ofaromatic species and of sulphur containing species. This might beexpected to lead to a lower natural antioxidancy and hence to a lowerstorage stability. In the past, it has often been thought necessary toblend Fischer-Tropsch derived fuels with other fuel components, and/orto process them in particular ways, in order to improve their storagestability (see for example U.S. Pat. No. 6,162,956 in which aFischer-Tropsch fuel is blended with a raw gas field condensatedistillate fraction or a mildly hydrotreated condensate fraction inorder to improve its oxidation stability, and WO-A-97/14768 andWO-A-97/14769 in which a high stability diesel fuel is prepared byseparating a Fischer-Tropsch derived fuel into two fractions, one ofwhich is hydrotreated prior to recombining with the non-hydrotreatedfraction).

At the same time, however, Fischer-Tropsch derived fuels also tend tocontain low levels of aromatic species and of cyclic paraffins, andrelatively low ratios of iso- to normal paraffins. It has now been foundthat, in the case of these particular fuel components, this appears tocounter the low inherent antioxidancy and results, overall, in increasedstorage stability. This in turn may be used to increase the storagestability of a fuel composition to which a Fischer-Tropsch derived fuelis added.

Preferred features of the eighth aspect of the present invention, forinstance the nature(s) of the fuel component(s) and optionally of anyadditives present in the fuel composition, and the nature andconcentration of the Fischer-Tropsch derived fuel component, may be asdescribed above in connection with the first to the fifth aspects of thepresent invention.

In particular, the Fischer-Tropsch derived fuel component preferably hasan olefin content of 0.5% w/w or lower, more preferably 0.1% w/w orlower. It suitably has an iso- to normal-paraffins ratio (i:n) of from3:1 to 4:1. It may have a kinematic viscosity at 40° C. of from 2.5 to4.0 mm²/s.

The concentration of the Fischer-Tropsch derived fuel component, in acomposition prepared according to the eighth aspect of the presentinvention, may also be as described above in connection with the firstto the fifth aspects of the invention. Suitably it may be from 5 to 30%v/v. In some cases the fuel composition may consist solely oressentially (for instance, optionally with one or more fuel additives)of the Fischer-Tropsch derived fuel component. Again, a mixture of twoor more Fischer-Tropsch derived fuel components may be used together inaccordance with the eighth aspect of the present invention.

This aspect of the present invention may additionally or alternativelybe used to adjust any property of the fuel composition which isequivalent to or associated with storage stability, for example toreduce its tendency to accumulate peroxides and/or acidic species and/orgums and sludges, and/or to reduce its corrosivity.

The storage stability of a fuel composition may in the present contextbe regarded as its oxidation stability, typically during normalconditions of storage and use. It may be assessed in any suitablemanner, such as by reference to the peroxide content of the compositionfollowing a fixed period of storage and/or use under specifiedconditions (peroxide content may be measured using standard test methodASTM D3703). Instead or in addition, storage stability may be assessedusing standard test method ASTM D2274 (oxidation stability byaccelerated method).

The terms “increasing” and “improving”, in the context of storagestability, embrace any degree of increase or improvement. The increasemay for instance result in the fuel composition having a peroxide levelwhich is at least 10% lower than that of the same composition withoutthe Fischer-Tropsch derived fuel component, after a specified period ofstorage under specified conditions. This figure may in cases be at least25 or 50 or 75 or 80 or in some case 90 or 95 or even 98 or 99%. Thespecified storage period may for example be 4 weeks or 8 weeks or 12weeks or 18 weeks, if the fuel is stored for example at 40° C. or higher(e.g. at 43° C. as in many standard fuel storage tests) or 60° C. orhigher. The storage period may be 2 years or more, for example from 2 to4 years, in particular if the fuel is stored under normal ambientconditions, for example at from 20 to 25° C.

The increase in storage stability may be as compared to the storagestability of the fuel composition prior to the realisation that aFischer-Tropsch derived fuel component could be used in the way providedby the present invention, and/or of an otherwise analogous fuelcomposition intended (e.g. marketed) for use in an analogous context,prior to adding a Fischer-Tropsch derived fuel component to it inaccordance with the present invention.

In absolute terms, the peroxide level of a fuel composition preparedaccording to the present invention is preferably 10 mg/kg or less, morepreferably 5 or 2 or even 1 mg/kg or less, after a period of storage ofone year under normal ambient conditions, and/or after a period ofstorage of 8 or 12 weeks under storage at 40° C. or higher.

A ninth aspect of the present invention provides a method forformulating a fuel composition, which method involves blending togethera non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derivedfuel component, optionally with other fuel components (such as fueladditives), for the purpose of increasing the storage stability of theblend. The method of either the eighth or the ninth aspect of thepresent invention may be used for the purpose of achieving a desiredtarget (typically minimum) level of storage stability for the fuelcomposition.

According to a tenth aspect, the present invention provides a method ofoperating a fuel consuming system, which method involves introducinginto the system a fuel composition prepared in accordance with theeighth or the ninth aspect of the present invention. The fuelcomposition may be introduced for one or more of the purposes describedabove in connection with the eighth and ninth aspects of the presentinvention, in particular to improve the storage stability of the fuelcomposition and/or to reduce occurrence of effects associated (whetherdirectly or indirectly) with fuel storage instability, for examplefilter blocking or valve or injector fouling, or increased sootproduction or increased corrosivity (to metals and/or elastomers).

Again a “fuel consuming system” includes a system which transports (forexample by pumping) or stores a fuel composition, in particular onewhich causes a physical disturbance to the composition (such as bypumping) which might serve to disperse sludges.

According to an eleventh aspect, the present invention provides a fuelcomposition preparable by, or which has been prepared by, a methodaccording to the eighth or ninth aspect of the present invention.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, mean “including but not limited to”, anddo not exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Preferred features of each aspect of the present invention may be asdescribed in connection with any of the other aspects.

Other features of the present invention will become apparent from thefollowing examples. Generally speaking the present invention extends toany novel one, or any novel combination, of the features disclosed inthis specification (including any accompanying claims and drawings).Thus features, integers, characteristics, compounds, chemical moietiesor groups described in conjunction with a particular aspect, embodimentor example of the present invention are to be understood to beapplicable to any other aspect, embodiment or example described hereinunless incompatible therewith.

Moreover, unless stated otherwise, any feature disclosed herein may bereplaced by an alternative feature serving the same or a similarpurpose.

The following examples illustrate the properties of fuel compositionsprepared in accordance with the present invention, in particular theirability to dissolve catalytic metals and their thermal and storagestabilities.

EXAMPLE 1

This example assessed the ability of four different automotive dieselfuel compositions to solubilise catalytic metals when in contact withmetal surfaces. The compositions were stored over a copper billet at 43°C. and atmospheric pressure, samples being taken monthly to determinetheir copper content by Inductively Coupled Plasma Mass Spectrometry(ICP-MS).

The fuels used were:

-   -   F1 a commercially available ultra low sulphur automotive diesel        fuel (petroleum derived), sourced in the UK;    -   F2 & F3 commercially available zero sulphur automotive diesel        fuels (petroleum derived), sourced in Sweden and Germany        respectively; and    -   F4 a Fischer-Tropsch derived gas oil (ex. Shell).

The four fuels had the properties listed in Table 1 below.

TABLE 1 Fuel property Test method F1 F2 F3 F4 Cetane ASTM D613 60.2 58.652.0 >74.8 number Density @ IP 365/ 0.8312 0.8112 0.832  0.7852 15° C.ASTM (g/cm³) D4052 Kinematic IP 71/ 2.041 2.86  3.606 viscosity @ ASTMD445 40° C. (mm²/s) Cloud point IP 219 −6 −34 −9.0  +2 (° C.) CFPP (°C.) IP 309 −36  (−1)  (+1) Distillation IP 123/ (° C.): ASTM D86 IBP171.8 188.8 172.2 211.5 10% 211.2 207.0 209.2 249.0 recovered 20% 230.7211.5 227.1 262.0 30% 250.5 219.8 243.8 274.0 40% 264.8 228.0 258.8286.0 50% 276.9 235.8 272.8 298.0 60% 287.9 243.2 287.0 307.5 70% 298.7250.6 301.8 317.0 80% 311.2 259.0 318.1 326.5 90% 328.1 270.3 338.8339.0 95% 345.2 279.3 354.2 349.0 FBP 358.7 290.3 363.7 354.5 SulphurASTM 39 <5 8.0  <5 content D2622 WDXRF) (mg/kg) Aromatics IP 391 (% m)(mod) Mono 3 22.1  0.1 Di <0.1 2.6  <0.1 Tri <0.1 0.3  <0.1 Total 3 25.0 0.1

The results of the copper solubilisation tests are shown in Table 2below.

TABLE 2 Copper content (ppbw) Sulphur Day Day Day Day Day Day Fuel(mg/kg) 0* 28 54 84 112 140 F1 39 17 80 160 500 880 810 F2 <5 4 50 80148 190 220 F3 <10 <3 20 30 93 170 190 F4 <5 <3 <20 15 34 60 95 (*=prior to storage) (ppbw = parts per billion by weight)

It is clear from Table 2 that the Fischer-Tropsch derived fuel F4 has asignificantly lower propensity to dissolve the copper than any of themore conventional, petroleum derived, diesel fuels.

EXAMPLE 2

In this example, the intrinsic thermal stabilities of the four fuels F1to F4 were assessed using the Jet Fuel Thermal Oxidation Tester (JFTOT),according to the standard test method ASTM D3241 (IP 323). Thistechnique, developed for the evaluation of jet fuels, involves pumpingfuel over a heated tube at a specified flow rate for a specified periodof time. The JFTOT “breakpoint” is the highest temperature (measured tothe nearest 5° C.) at which the fuel passes the JFTOT test criteria,which relate to tube appearance and test filter pressure differential.The JFTOT test was chosen as it subjects a fuel to higher temperaturesthan those typically observed in a diesel engine, and thus provides arelatively stringent assessment of a fuel's stability. It can also,being an accelerated test method, yield stability data in a relativelyshort time period.

The results for the four fuels are shown in Table 3.

TABLE 3 JFTOT Sulphur breakpoint Fuel (mg/kg) (° C.) F1 39 240 F2 <5 350F3 <10 285 F4 <5 >380

Table 3 shows that the Fischer-Tropsch derived fuel F4 is significantlymore thermally stable than any of the petroleum derived diesel fuels,even the zero sulphur diesels F2 and F3 which have comparable levels ofsulphur. Even when tested at 380° C. (the highest temperature achievableusing the JFTOT), the Fischer-Tropsch fuel still passed the testcriteria.

EXAMPLE 3

This example assessed the impact of copper pick-up on the thermalstability of diesel fuels. Fuels F2 to F4 (those having comparably lowsulphur levels) were assessed using the JFTOT method as outlined inExample 1, after doping with an appropriate quantity of coppernaphthenate. The doping levels were chosen in each case to approximateto those found in the fuels after 8 weeks' storage in contact with acopper billet, as observed in Example 2. Thus, 50 ppbw of copper wasaimed for in fuels F2 and F3, this level being midway between the 80ppbw and 30 ppbw that were respectively detected in these fuels at day54. For the Fischer-Tropsch derived fuel F4, a dosing level of 20 ppbwwas aimed for.

The JFTOT results are shown in Table 4.

TABLE 4 Cu Cu content content Neat fuel Cu-doped (ppbw) - (ppbw) - JFTOTJFTOT Sulphur target measured breakpoint breakpoint Fuel (mg/kg) levellevel (° C.) (° C.) F2 <5 50 55 350 345 F3 <10 50 60 285 220 F4 <5 2015 >380 >380

As seen in Table 4, the Fischer-Tropsch derived fuel F4 still hadexcellent thermal stability despite the copper which it might forinstance have dissolved after 8 weeks' contact with a copper-containingsurface. After storage under similar conditions, the two petroleumderived diesel fuels have taken up significantly more copper and thisappeared to have affected their thermal stability, fuel F3 in particularshowing a significant decrease in its JFTOT breakpoint compared to theneat fuel.

Thus, even when exposed to catalytic metals during storage, and/orduring use in a fuel consuming system such as a diesel engine, aFischer-Tropsch derived fuel component appears less likely to sufferfrom a reduction in thermal stability than is a petroleum derived dieselfuel.

A Fischer-Tropsch derived component may therefore be incorporated into afuel composition, according to the present invention, in order to lessenits metal pick-up susceptibility and hence improve its thermalstability.

EXAMPLE 4

This example assessed the storage stability of five different automotivediesel fuel compositions, with reference to their tendency to accumulateperoxides during storage.

The compositions were stored at 43° C. and atmospheric pressure, in air,for 24 weeks. Samples were taken at monthly intervals to determineperoxide content, using a modified version of ASTM D3703 so as to avoidthe use of halogenated solvents. The relatively high storage temperaturewas intended to mimic longer storage periods under normal ambientconditions.

The fuels used were:

-   -   F1 & F2 commercially available ultra low sulphur (<50 mg/kg)        petroleum derived automotive diesel fuels, both sourced in the        UK;    -   F3 a commercially available “zero sulphur” (<5 mg/kg) petroleum        derived automotive diesel fuel, sourced in Sweden; and    -   F4 & F5 two Fischer-Tropsch derived gas oils (ex. Shell), both        with sulphur contents of <5 mg/kg.

Their aromatics contents were between 20 and 30% m for F1 and F2, lessthan 5% m for F3 and <0.5% m for the Fischer-Tropsch derived gas oils F4and F5.

The peroxide contents of the extracted fuel samples are shown in Table 5below.

TABLE 5 Peroxide content (mg/kg) Sulphur 0 4 8 12 20 Fuel (mg/kg) weeksweeks weeks weeks 18 weeks weeks 24 weeks F1 45 0.2 3 0.6 23.7 21.3  NT27.2  F2 <50 0.3 0.6 0.6 49.7 0.3 NT 0.8 F3 <5 0.1 0.05 0.9 50 0.3 NT0.1 F4 <5 0.3 0.9 0.9 0.4 NT 1.3 NT F5 <5 0.2 0.3 1.6 0.6 NT 0.9 NT (NT= not tested)

The Table 5 data show fluctuations in peroxide levels throughout thestorage period, as a result of both the test methodology and the factthat peroxides can themselves decay to other oxidation products.Nevertheless, overall the data show that for the conventional petroleumderived diesel fuels F1 to F3, peroxide levels increase significantlyafter only eight to twelve weeks' storage. Those for the Fischer-Tropschderived gas oils F4 and F5, however, remain low (at effectively thedetection limit of the test method) throughout a 20 week storage period.This indicates a far higher oxidation stability for the Fischer-Tropschderived fuels.

EXAMPLE 5

As discussed above, a number of factors are now believed to influencethe storage stability of a fuel. These include not only the degree ofnatural antioxidancy inherent in the fuel, but also its hydrocarbonstructure. The ability to form stable hydrocarbon radicals will promoteradical driven autoxidation processes and hence decrease the storagestability of a fuel. Radical stability is believed to decrease in theorder aromatics>cyclic and iso-paraffins>normal paraffins.

Table 6 below compares the composition of the Fischer-Tropsch derivedgas oil F5 used in Example 4 with that of a commercially availablepetroleum derived ultra low sulphur diesel fuel F6, sourced in the UK.

TABLE 6 Composition (% w/w) Component F5 F6 Normal and iso- 99.77 43.84paraffins Cyclic paraffins 0.22 24.55 Dicyclic paraffins 0.00 8.46Mono-aromatics 0.01 18.29 Di- & poly- 0.00 4.86 aromatics Total 100.00100.00

Overall, it can be seen that the petroleum derived fuel F6 has a farhigher concentration of the fuel components (for example, aromaticspecies and cyclic paraffins) which are likely to be able to form stableradicals and hence promote autoxidation. The Fischer-Tropsch derivedfuel, in contrast, contains only a trace of cyclic paraffins andvirtually no aromatic components, its composition being mainly normaland iso-paraffins. This means that the fuel will form much lower levelsof stable radical species, which in turn is believed to contribute toits significantly higher storage stability.

A Fischer-Tropsch derived fuel may therefore be used, in accordance withthe present invention, to improve the overall storage stability of afuel composition into which it is incorporated.

1. A method for formulating a fuel composition, which method comprisingblending together a non-Fischer-Tropsch derived base fuel and aFischer-Tropsch derived fuel component, optionally with other fuelcomponents, in which the JFTOT breakpoint of such fuel composition isgreater than 300° C.
 2. The method of claim 1 which the JFTOT breakpointis greater than 350° C.
 3. The method for formulating a fuelcomposition, which method comprising blending together anon-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived fuelcomponent, optionally with other fuel components, the peroxide level ofsuch fuel composition is 10 mg/kg or less after a period of storage of 8weeks under storage temperature of at least 40° C.
 4. A method ofoperating a fuel consuming system, which method involves introducinginto the system a fuel composition prepared according to claim
 1. 5. Amethod of operating a fuel consuming system, which method involvesintroducing into the system a fuel composition prepared according toclaim
 2. 6. A method of operating a fuel consuming system, which methodinvolves introducing into the system a fuel composition preparedaccording to claim
 3. 7. The method of claim 2 wherein the concentrationof the Fischer-Tropsch derived fuel component in the fuel composition isfrom 5 to 30% v/v.